By Glenn Dobbs, VAF Filtration Systems, USA
And Roy McDougall, Hydrochem, Australia

The high return on resources (ROR) provided by cooling tower filtration systems (CTFS) can efficiently reduce maintenance and meet green building Initiative requirements.

As water quality continues to decline, water costs escalate. For these reasons, many companies are realizing that the cost of a well-designed CTFS often is small compared to the high cost of cooling system maintenance resulting from poor water quality, and that the once accepted practice of discharging large amounts of wastewater to the sewer is not feasible.

General
Keeping an open recirculation cooling water system clean assists in maintaining heat transfer efficiency, reduces maintenance and enhances the effectiveness of the water treatment program. Open systems are prone to fouling from airborne, water borne and contaminants associated with a process. Systems can also produce suspended solids from within due to corrosion, scale and microbiological growth.

Separators, media filters, automatic screen filters and cartridge/membrane systems all offer an effective way of removing material from the circulating water; however each of these filtration technologies offer differing benefits and associated costs. Understanding these technologies’ differences is necessary for proper selection.

Technology selection should be made in conjunction with a specialist and/or a filtration system manufacturer depending on the:
  • Quality of make-up water.
  • Type of contaminants getting into the system.
  • Type of contaminants being generated within the system.
  • Operational duty of the system.
  • Type of water treatment being applied.
  • Cycles of concentration at which the system is being operated.
Benefits of Cooling Tower Filtration
Incorporating an efficient CTFS into a cooling tower can extend the cooling tower system’s life including pipework, chillers and plate heat exchangers, and lower maintenance costs while improving cooling efficiencies, thereby providing a significant ROR by:
  • Improving cooling system efficiencies.
  • Increasing life of the cooling system.
  • Reducing treatment chemical costs.
  • Reducing maintenance.
  • Reducing cleaning downtime.
Contamination Sources
Three primary sources for common cooling tower contaminants:
  • Ambient air dirt and windblown sand, soot and organic debris.
  • Calcium carbonate scale forms in the cooling tower system and flakes off. Chemical residue and algae also build up in the circulation water.
  • Depending on the source and water quality, the makeup water can contribute to contaminant buildup.
A side-stream CTFS can be sized to filter 5 to 10 percent of the full stream, which will clean and stabilize the amount of particulate load in a short amount of time.

Filtration Technologies
Several different technologies are used to filter cooling tower water. These include:
  • Centrifugal separators.
  • Sand media.
  • Automatic self-cleaning screens.
  • Cartridges, bags, membrane.
Centrifugal separators removes up to 90 percent of the particles that have a specific gravity greater than water and are larger than 70 microns.2Organics and small particles remain.

Sand media filters require a large footprint. Media flush waste is high compared to other filtration technologies; i.e., a sand media system that requires 1,200 gallons to flush, where some screen filter technologies use only 30 gallons to flush.

Micro-filtration is only required for specialized cooling systems; cartridge, bag or membrane systems can remove organic and inorganic particles down to 0.5 micron without requiring back-flushing. These technologies can be expensive; cartridges, bags and membrane can foul and require frequent replacement.

Automatic self-cleaning screen filters are becoming the filter technology of choice for CTFS applications. This technology removes organic and inorganic particulate down to the micron rating of the filter. These filters provide the least wastewater during the cleaning process. With lower flush volume required to clean the screen, it’s much easier to show that none of the flush water is waste since it is a small portion of blow down requirements.3

Basin Cleaning Systems
CTFS can be installed directly at the cooling tower basin to remove contaminants directly from the primary source. Contaminants from the air and suspended solids in the circulating water tend to settle out in the basin. Incorporating water jets in the basin creates turbulence that direct solids toward the filter’s intake increasing filtration efficiencies.

System Design
CTFS can be designed to meet any level of filtration quality and should consider these features:
  • Removal of organic and inorganic particulate.
  • Uninterrupted filtration during flush.
  • Flush flow rates of 5 to 10% of total flow rate.
  • Short cleaning cycles.
  • Simplicity: Fewer moving parts.
  • Efficient filtration: 80-100um filtration is the most common for cooling tower water. Required filtration might vary depending on the tower location, local conditions and filtration objective.
Most importantly, purchase the level of CTFS that will achieve the highest ROR; therefore will give you the greatest ROI.

Reference
1. ASHRAE 2000 Systems and Equipment Handbook 2. Based on published separator manufacturers’ statistics. 3. Cooling Tower Filtration Efficiencies and Green Building Initiatives, by Glenn Dobbs, VAF Filtration Systems, 2010

Glenn Dobbs: President VAF Filtration Systems, Arvada, CO
Roy McDougall: Senior Projects Manager, Hydrochem; Australia
For a copy of this entire article, please contact VAF
PH +1 303 425 4242
Web www.valveandfilter.com
Email sales@valveandfilter.com


Links

 

*In 2016, VAF Valve and Vilter, Arvada, Colo., was acquired by Evoqua. Learn more about VAF at https://www.evoqua.com/en/brands/VAF-Filtration/.